The incredulity fallacy is the fallacy of judging a belief, or argument, based on one's ability to understand or believe it. It occurs in two basic forms: "I can't believe it's true, so it's false", or "I can't believe it's false, so it's true".
It's somewhat unusual for the fallacy to be phrased exactly as above, though especially at the popular level, you will find many such examples. But it is more often disguised as an "objective" statement. "Biology makes no sense except in light of Evolution", for example. This does not immediately sound like an expression of incredulity, because it doesn't say "I can't understand how Evolution is false", or "I can't believe non-Evolution worldview X is true". However, the statement "biology makes no sense except in light of Evolution" is a veiled expression of incredulity framed as an objective statement.
To illustrate this, simply flip the elements: "Evolution makes no sense in light of biology". Of course, as a firmly convinced Christian, I agree with the sentiment. However, it would be the incredulity fallacy to argue that way. Let's break it down further: "Biology makes no sense to me except in light of Evolution". By adding those two simple words, I have removed a key assumption: That anyone else should share my incredulity. But suddenly, the statement doesn't sound as objective.
This, of course, is because it isn't. Setting aside the obvious fact that there are a great many people, both laymen and qualified biologists, who find biology makes perfect sense without any reference to Evolution, but even if, somehow, no one could fathom that Evolution is false, that wouldn't make it true.
It's important to note that the mere expression of incredulity, which we're all prone to do, is not, in and of itself, the incredulity fallacy. Rather, one's own strength of belief must be included as a premise. In the above arguments, the logic is as follows:
Premise 1: I don't understand how biology works without Evolution.
Premise 2: Biology cannot work in a way I don't understand.
Conclusion: Evolution is true.
Alternatively:
Premise 1: I don't understand how biology works in light of non-Evolutionary view A.
Premise 2: Biology cannot work in a way I don't understand.
Conclusion: Biology does not work in light of non-Evolutionary view A.
There are, however, ways to express incredulity alongside an argument. For example, "given that we constantly observe organisms change over time, I can't understand why you don't believe in Evolution".
The logic of the above statement, while of course still flawed (as is repeatedly demonstrated in the Evolution section of this site), does not depend on the element of incredulity, which is actually more of a conclusion. The argument here is:
Premise 1: We constantly observe organisms changing over time.
Premise 2: This is experimental proof of Evolution.
Conclusion: Evolution is true.
In the above argument, the incredulity is the conclusion, not the argument itself, and it is entirely acceptable to use solid arguments to back up your own incredulity.
I should mention, at this point, that I quite deliberately chose this example. If you type in "incredulity fallacy" to a search engine, you will find a number of logic sites which explain the fallacy in great detail. But one thing I notice is that while these sites typically frame themselves as objective, they frequently use Creationism as their examples. In fact, the example in the header image is directly stolen, and altered, from such an article. The example they give is "“I can’t imagine how humans could have evolved from single-celled organisms; it just doesn’t make sense to me. There is no way that the theory of evolution is right.”" I merely altered it to read "“I can’t imagine how humans could be descended from Adam and Eve. It just doesn’t make sense to me. There is no way that the Bible is true.”"
Now, on one hand, I have no problem with genuinely fallacious Creationist arguments being presented as examples. I want my fellow Creationists to make good, solid arguments, and avoid the fallacious ones. This requires thinking critically about our own view. But it strikes me as suspicious that "objective" sources, whose stated goal is to help their audience think critically, would be so imbalanced in their provision of examples.
Now of course, I do similar things. When I write an article explaining a fallacy, the examples I give are always non-Christian arguments either against Christianity, or in favor of non-Christian views. But there are two key differences. The first difference is that you probably know what you're getting into when you come to a site called "Bible Brain". I am biased towards the Christian faith, and I don't intend to hide that. But more importantly, because I am confident in my views (and want that confidence shaken if it turned out they weren't true) that I have no issue thinking about them critically, and actively encourage others to do likewise.
With that in mind, I actually fully support the original example. It is a true example of the incredulity fallacy to say "I can’t imagine how humans could have evolved from single-celled organisms; it just doesn’t make sense to me. There is no way that the theory of evolution is right." Therefore, it is essential for every Creationist (or indeed anyone who finds Evolution unbelievable) to avoid that logic. But it is likewise the incredulity fallacy for an Evolutionist to say “I can’t imagine how humans could be descended from Adam and Eve. It just doesn’t make sense to me. There is no way that the Bible is true.”
No matter how hard I try, I cannot find a single example of an article on the incredulity fallacy presenting an Evolutionist as the example. Yet, so far, they all present Creationism. Even Wikipedia, that everlasting fountain of knowledge, says that arguments from incredulity "...are also frequently used to argue that something must be supernatural in origin." But there is no attempt to warn the reader that Naturalists are just as prone to make such arguments.
With this in mind, while the primary purpose of this article is to explain the incredulity fallacy, to the extent that my readers will be able to spot it whenever you come across it, I want to throw in two bonus lessons.
The first is to be aware that even the most apparently objective sources may be attempting to brainwash you. This could even be unintentional. Deceived people deceive people. Thus, just because a source claims to be objective and unbiased doesn't mean it is.
But second, because I want my readers to learn to think critically, I want them to learn to think critically about everything, whether it be their views, or even mine. Thus, an important phrase to learn is "what's good for the goose is good for the gander". Whenever you read examples of a fallacy, consider opposing views, too, and see if they may make the same.